[Cython] Dropping support for Python 3.2

[Stefan Behnel]

Hi all,

the release branch still builds ok with Python 3.2, but master doesn't, and
I'd rather not spend my time on fixing that. Python 3.2 has reached its
final end-of-life in February 2016, so it's really dead for good now.

https://www.python.org/dev/peps/pep-0392/

I'm not really expecting any objections. ;) Unless someone stands up to
invest time into making all tests pass in Py3.2 again, I'll rip out the
Py3.2-only usage of 2to3 from setup.py during the next weeks, and also the
small set of special cases here and there in the C code.

Stefan